Such a method, or apparatus therefor, respectively, has become known from the GB-PS 1 393 061. In this known arrangement, the bulk material is on a transport ribbon, above which a video camera is provided, and at the end of which an array of nozzles that can be actuated individually are arranged across the width of the transport ribbon. Differences in luminosity of the particles on the transport ribbon are detected with the help of the video camera, and, via a comparator circuit, one of the nozzles is actuated with a corresponding time delay to eject the respective particle out of the path of the bulk material, thereby sorting it out. A disadvantage of this arrangement is provided by the fact that it is often not possible to eject the very particle to be sorted out by means of the gas jet acting on the particles falling off the transport ribbon in a free fall. For this reason, it was unavoidable to operate the air nozzles during a longer period of time than it would have been necessary as such, which, in turn, resulted in the ejection of more particles than required, that is, among the particles of an unacceptable quality that had been sorted out there was to be found also a high percentage of such particles of an acceptable quality.
From the DE-A-37 01 335 a method for sorting particles of a bulk material has become known, initiated by a presorting step, by means of which a part of the bulk material containing particles to be sorted out, is segregated. Also in such a case, that part of the bulk material that has been segregated (similarly to the GB-PSen 1,546,548 or 2,091,415) is passed, while falling freely, by an electro optical monitoring unit that detects the particles to be sorted out and controls an air nozzle, by means of which the particles falling down freely are deflected from their path. It is true that mechanically actuable flaps were used for this presorting step, but these were not capable of conducting a precise sort, for which reason a second sort was required.
It is an object of the invention to provide an apparatus for sorting particles of a bulk material, which apparatus ensures an exact ejection of the particles to be sorted out, even in such cases where the individual particles of the bulk material are of different sizes; in particular, by means of the apparatus provided by the invention, the chronological relationship between the detection of an imperfect particle and the operation of the pneumatic ejection unit is to be better coped with, thus enabling an improved sorting accuracy.